Photodetection device and method

ABSTRACT

The photo-detecting device includes a photodetector for detecting incident light, an input J FET for reading the sensing signal from the photodetector, an amplifier for amplifying the signal detected by the input J FET, a feed-back circuit for feeding the output of the amplifier back to the gate of the input J FET through a feed-back capacitor, a reset circuit for resetting the feed-back capacitor by discharging it with a reset MOS FET, and a circuit of a switch and resistor. The same level voltage as the gate voltage of the input J FET is applied to its source through a resistor, and the circuit of the switch and the resistor is connected between the source of the reset MOS FET and the feed-back capacitor. The reset MOS FET and switch are controlled so that the reset MOS FET is turned “on” and “off” while the switch is “on”.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/394,247,filed Mar. 24, 2003, allowed. This application is based upon and claimsthe priority of Japanese Patent Application No. 2002-85102, filed Mar.26, 2002 in Japan, the contents of which are incorporated herein forreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to ahypersensitive photodetection device in which a CTIA (capacitivetrans-impedance amplifier) or CIA (capacitive impedance amplifier) isapplied to the detection of feeble incident light such as is found influorescence observation in chemistry and weak-light observation inastronomy.

2. Description of the Related Art

The sensitivity of infrared-ray sensors, i.e. sensors used to detectinfrared rays, is increasing as semiconductor infrared-ray sensors andrelated peripheral-device techniques evolve. Two-dimensionalinfrared-ray sensor elements with detected noise levels of ten-oddelectrons have been developed.

The detection noise of two-dimensional infrared-ray sensors has beendecreased through reduction of the noise occurring in the detecting MOSFETs (metal oxide semiconductor field effect transistors), and theirinput capacitance and leakage current have been lessened, as well asthrough improvements in the performance of the two-dimensional infraredray sensor. That is, for a given amount of photocurrent in thephotodetector, a smaller input capacitance leads to a higher input gatevoltage, and less leakage current leads to lower levels of shot noise.

MOS FETs are used in the readout circuits of two-dimensionalinfrared-ray sensors because of the small characteristics-dispersion,leakage current, and input capacitance of a MOS FET. However, the noiselevel of a Si J FET (silicon junction field effect transistor), is about1/100 that of a MOS FET, while its input capacitance is in the rangefrom a few to ten times as high.

On the whole, however, the low-noise Si J FET is expected to improve theS/N over that for a MOS FET. Actually, if we only consider the currentlyknown forms of noise in a Si J FET, such as thermal noise andgeneration-recombination noise etc., we would expect to be able toobtain measured noise at the single-electron level in the band aroundthe 10-Hz range.

However, success in the measurement of such a noise level has not beenreported. Contradicting our expectations, one report stated that the lownoise level seen when a J FET is used in an amplifier that has a lowinput impedance becomes several times higher when it is used in anamplifier with a high input impedance. This noise level is notexplicable in terms of the shot noise of the leakage current. Theincrease in noise has been vaguely assumed to be because the storagenoise is strengthened by the increase in input impedance.

Studies by the inventor of the present invention have shown that thedielectric polarization noise is dominant in the high input impedancecase. The polarization noise is caused by the phenomenon of the thermalfluctuation of polarization, which is derived from thefluctuation-dissipation theorem as well as Johnson noise is derived fromthe theorem.

The polarization noise is the principal limit on the noise, because thisnoise is inevitable in a photodetector and J FET used as a sensor, thatis, the noise is inherent to the materials. The detected noise of theprior-art photodetection device has not approached the theoreticallylimiting range of noise levels because of noise generated by the leakagecurrent or and other noise sources of the FET.

FIG. 7 shows an example of the circuit for a photodetector of the priorart. A CTIA circuit, which is an orthodox circuit and in generally usein photodetection devices, is used in the photodetection device of theprior art shown in FIG. 7.

The CTIA circuit is a TIA (Trans-Impedance Amplifier) in which acapacitor replaces a resistor of the feed-back circuit. A resistorinduces thermal noise which determines the limit on the detection oflight, but the capacitor in the feed-back loop induces little thermalnoise and improves the limit on the detection of light.

However, the photocurrent does not disappear outside the circuit, andcharges up the capacitor. We thus need to evacuate the accumulatedcharge with appropriate timing. This action is called a reset, while thestep of measuring the photocurrent is called carrier accumulation(charging of the capacitor).

The photo detection circuit consists of the photodetector that detectsincident light (for example, a photodiode) 1, a J FET 2 for reading outthe detected light the detection of light, an op amp. (operationalamplifier) 3 that amplifies the detection signal, the feed-back loopthat feeds the output of the op amp. 3 back to the gate of the J FET 2through the capacitor 4, and the reset circuit that resets the capacitor4 by discharging the capacitor 4 through a MOS FET, S1.

Furthermore, the photodetector (for example, a photodiode) 1, input JFET 2 for readout, capacitor 4 and MOS FET S1 are placed in a cryogenicvessel which is cooled down to a cryogenic temperature (for example77K), while the amplifier 3 is placed at room temperature.

In this case, the elements in the cryogenic vessel are connected withthe op amp. 3 by conductors. Also, the reset pulse to reset the MOS FETS1 can be applied with a control circuit (not shown in the figure)situated outside the vessel, with conductors connecting the MOS FET S1and this control circuit.

In the above circuit, a feed-back capacitor 4 replaces a resistor of theTIA circuit which is generally used in infrared-ray readout circuits. Togive the TIA circuit a large S/N, as large a resistance value as ispossible is chosen for the resistor of the overall TIA circuit'sfeed-back circuit. Johnson noise is thus the dominant form in whichnoise is generated, and this prevents the detection of infrared rays.When the feed-back capacitor 4 is used in the feed-back loop, thephotocurrent provides charge that accumulates in the feed-back capacitor4, so the CTI circuit is provided with the MOS FET 1 for resetting as areset switch that discharges the feed-back capacitor 4.

Furthermore, since a very high input impedance is needed to detect theweak radiance of infrared rays, an FET is used in the input circuit. AnFET can operate at low temperatures, and can thus be placed very closeto the cooled sensor; that is, the length of the high-impedance portionis wired to a short length. While the op amp. is placed at roomtemperature, all devices other than the OP amp. are set in the cryostatof liquid nitrogen.

The circuit contains noise sources of various kinds that originate inthe device elements. However, if we consider the behavior of thecircuit, the noise sources can be classified into two types. One typecovers the noise voltages generated in the source circuit of the J FET2, which is the input FET. This noise includes all noise generated inthe channel of the J FET 2 and the input-referred noise of the OP amp.Both noise voltages and noise current are referred to as input noise ofthe OP amp.

However, as the current noise is converted to voltage by the outputimpedance of the input J FET 2 for readout (henceforth referred to asthe input J FET), the current noise can be included with the voltagenoise. The noise when converted to current noise at the gate of theinput J FET 2 is to be compared with the photo current. Theinput-referred noise current to the input J-FET can be obtained bydividing the noise voltage with input impedance of the J-FET. S/N of thephotodetctor is measured from comparing the photocurrent of thephotodetector with the input-referred noise current.

The other kind of noise is that which flows directly into the gatecircuit of the input J FET 2, for example the shot noise of the leakagecurrent of the photodetector and the input J FET 2 for readout, andgate-induced noise of the JFET 2. The polarization noise of devicesconnected to the gate circuit is also of this kind. Noise of this kindis converted to input current noise, and is thus referred to as gatecurrent noise.

The respective two kinds of noise mentioned above can be measured byusing the dependence of input impedance. The noise is converted to thereferred noise voltage to op amp. 3 output by multiplying the feed-backimpedance to the input referred noise current. Thus referred noisevoltage of the source noise to op amp. 3 output is proportional to theratio of the input impedance to the feed-back impedance, while thecurrent noise at the gate is proportional to the feed-back impedancealone.

Thus, lowering the input impedance and feed-back impedance such that theratio is kept constant reduces the current noise at the gate tonegligible levels. On the other hand, when both of the impedances areincreased, the current noise returns to measurable levels.

In the prior-art photodetection device, the MOS FET S1 is connected inparallel with the feed-back capacitor 4. In this connection, the voltagebetween the electrodes of the feed-back capacitor 4 is applied directlyacross the source and drain of the MOS FET S1, and this induces aleakage current between the source and drain of the MOS FET S1.

Thus, even when the MOS FET S1 is off, the flow of some leakage currentbetween the source and drain is inevitable whenever any voltage isapplied across them. Thus, the noise is not reducible in spite of theuse of the feed-back capacitor 4 in the photodetection device; thisprevents high sensitivity in photodetection.

When a photodetector that has a larger light-incident area is used sothat more light is received, the capacitance of the photodetectorinevitably increases. On the other hand, the limit on the sensitivity ofphotodetection is determined by the noise level of the input J FET thatreads out the detection signal. The input-referred noise of the input JFET is proportional to the capacitance of the photodetector; however,the polarization noise is proportional to the square root of thecapacitance. Thus, as the capacitance of the photodetector is increased,the input-referred noise of input J FET becomes dominant. Reducing thenoise level to the limit imposed by polarization noise is thus difficultwith the prior-art photodetection device.

SUMMARY OF THE INVENTION

One objective of the present invention is to solve the problems with theprior art and reduce the noise in the photodetection device by as muchas is possible, thus improving the sensitivity of photosensing. Thepresent invention consists of the following constructions for solvingthe problem.

Photodetection device, that is a photo-detecting device, A of thepresent invention, which corresponds to FIG. 1, includes a photodetectorfor the detection of incident light, an input J FET for reading out ofthe detection signal from the photodetector, an amplifier for amplifyingthe signal detected by the input J FET, a feed-back circuit that feedsthe output of the amplifier back to the gate of the input J FET througha feed-back capacitor, a reset circuit that resets the feed-backcapacitor by discharging its charge through a MOS FET (henceforthreferred to as a reset MOS FET), and a circuit made up of a switch andresistors. The same voltage as is at the gate of the input J FET isapplied to the source (or drain) through a resistor, and the circuitcomposed of the switch S2, and resistor is connected between the source(or drain) of the reset MOS FET (which corresponds to the MOS FET S1 inFIG. 1) and the feed-back capacitor. The reset MOS FET and the switch S2are controlled so that the reset MOS FET is turned “on”, while theswitch 2 is “on”. That is, the reset MOS FET is turned “on” and the“off”, when the switch S2 is “on”. After that the reset MOS FET isturned “off”, the switch S2 is turned “off”. Resetting of the feed-backcapacitor is carried out while the reset MOS FET and the switch S2 are“on”.

The reset circuit is thus only inserted in the feed-back loop at thetime of a reset, and the source (or drain) of the reset MOS FET can bekept at the same voltage as the gate of the input J FET while thefeed-back capacitor is being charged.

In the prior art, on the other hand, the reset MOS FET is configured inparallel with the feed-back capacitor. The voltage applied to thefeed-back capacitor is thus applied directly across the source and drainof the reset MOS-FET, and induces a leakage current. This leakagecurrent is inevitable whenever a voltage is applied across the sourceand drain, even when the reset MOS FET is “off”. In the presentinvention, on the other hand, this leakage current is avoided by havingthe source (or drain) of the reset MOS-FET at the same voltage as thegate of the input J FET.

Moreover, applying the switching circuit to the reset circuit allows usto separate the reset MOS FET from the output of the amplifier duringcharging of the feed-back capacitor. In addition, the reset MOS FET isconnected to a resistor so as to avoid large fluctuations of voltage atthe source (or drain) of the reset MOS FET during a reset.

A certain gate voltage, which is decided from the source (or drain)voltage of the reset MOS FET, has to be applied to make the reset MOSFET turn “on” and reset the feed-back capacitance. The source (or drain)of the reset MOS FET is connected to the output of the amplifier, sothat the source voltage varies with the output voltage of the amplifier.However, connecting a resistor between the source (or drain) and theoutput of the op amp. keeps variation at the source (or drain) of thereset MOS FET from varying greatly, even when the output voltage isvaried. The reset MOS FET is thus controllable regardless of the voltageoutput by the amplifier. In the above explanation J FET is used as aninput element for reading out of the detection signal of thephotodetector. But other FETs such as MOS-FET etc. are usable as theinput element.

The photodetection device, that is a photo-detecting device, B of thepresent invention corresponds to FIG. 3 is comprised of a photodetectorfor the detection of incident light, an input J FET for reading out ofthe detection signal from the photodetector, an amplifier for amplifyingthe signal detected by the input J FET, a feed-back circuit to feed theoutput of the amplifier back to the gate of the input J FET through thefeed-back capacitor, a reset circuit that resets the feed-back capacitorby discharging its charge through a pn-junction element for resettingthe capacitor (henceforth referred to as a reset pn-junction element)and the circuit made up of switch S2 and a resistor. The same voltage asat the gate of the input J FET is applied to the reset pn-junctionelement through a resistor, and the circuit made up of switch S2 and aresistor is connected between the reset pn-junction element and thefeed-back capacitor. The circuit has feature that, following turning thesecond switch to “on”, the first switch turns automatically to “on”.

The polarization noise of a MOS FET is not generally so weak. Thus, whena MOS FET is used as the reset switch, the polarization noise of thisMOS FET is dominant in the noise of the photodetecton device, and thedetection limit of the photodetection device is decided by the noise ofthe MOS FET. Using the pn-junction element instead of the MOS FET in theresetting circuit reduces the polarization noise.

Furthermore, the reset pn-junction does not need the reset pulse whichis required to make the reset MOS FET of the photodetection device Aturn “on”. The circuit can thus be simplified. In addition, applying thesame technique to the photodetector by using the reset pn-junctionelement eases fabrication of the photodetection device.

The photo-detecting device, that is a photo-detecting device, C of thepresent invention corresponds to FIG. 4 has the same structure asphotodetection device B except that the input J FET, reset pn-junctionelement, and switch S2 are made of a compound semiconductor, such asGaAs semiconductor. The action of the photodetection device C is alsothe same as that of the photodetection device B.

In the photodetection device C, too, the polarization noise is reducibleby using a compound-semiconductor pn-junction element rather than areset MOS FET as the reset switch for resetting of the photodetectiondevice A.

Following turning the switch S2 to “on”, the reset pn-junction elementturns automatically to “on”. The reset pulse is not needed thus, and thecircuit is simplified. Making the pn-junction switch S3 with the samematerials such as compound semiconductor with the material of J-FET 2 isconvenient, because the pn-junction switch S3 and the J-FET areintegrated to an integrated circuit. Further, the material of S3 isselectable from the same materials with the pn-junction element 3 andinput J-FET 2. In this way, input J-FET 2, pn-junction element 2 andswitch S2 are integrated as an integrated circuit. This makes thefabrication of photodetection devices of the present invention moreeasy.

Photo-detecting device, that is a photo-detecting device, D of thepresent invention, which corresponds to FIG. 5, is comprised of aphotodetector for the detection of incident light, a J FET for readingout of the detection signal from the photodetector, a reset pn-junctionelement which is connected to the gate of the input J FET, and a switchto turn the reset pn-junction element “on” and “off”. The resetpn-junction element S3 resets the photodetector 1 by discharging thepn-junction capacitor of the photodetector and/or the gate capacitor ofthe input J FET 2. The switch S3 is controlled by a controlling meansnot shown in FIG. 5.

Using the pn-junction element as the reset switch of the reset circuitin this way is applicable to the CIA circuit of the photodetectiondevice D as well as to the CTIA circuits of the photodetection devices Band C. Moreover, the circuit structure is simplified since a reset canbe performed by simply applying forward bias to the pn-junction elementfor resetting.

The structure of photodetection device, that is a photo-detectingdevice, E is comprised of multiple photodetectors. The area of thephotodetecting chip is 1/n of the chip area of the photodetecting chipof the above mentioned photodetectors. Further the n photo-incidentplanes are electrically connected in series with each other. Forexample, the photo-incident planes of each of the photodetectors are setin parallel each other, and the area of each of the photo-incidentplanes is S/n, where S is the area of all of the photo-incident planesand is the photo-sensing plane area of the set of photodetection devicesA-D (see FIG. 6), each of which is henceforth referred to as a standardphotodetector.

The total light-incident area of the photodetectors is equivalent to thelight-incident area of the standard photodetector. However, thephotocurrent of the photodetection device E is 1/n that of the standardphotodetector, and the total capacitance of the photodetection device Eis C/n², where C is the capacitance of the standard photodetector.Furthermore, as the input-referred noise of J FET is proportional to thecapacitance of the photodetector, the noise of the photodetection deviceE is reduced to 1/n² that of the given photodetection device A-D used asthe standard photodetector.

However, the level of polarization noise is 1/n because the polarizationnoise is proportional to the square root of the capacitance. So,increasing n more strongly reduces the input-referred noise of the J FETthan the photocurrent, thus improving the S/N ratio of thephotodetection device. Furthermore, when increasing the n, theinput-referred noise level of the J FETs comes to equal to thepolarization noise level at a “n₀” of “n”. Increasing the n from “n₀”furthermore, the input-referred noise level of the J FETs comes lessthan the polarization noise level, the polarization dominates theoverall noise. In the situation that the input-referred noise level ofthe J FETs is less than the polarization noise level, the noise levelfalls by 1/n as n increases. As the photocurrent is reduced at the rateof 1/n, the S/N ratio of the photodetection device E does not change.

Moreover, in the case where the shot noise of the leakage current isdominant, 1/n of the leakage current lowers the level of shot noise by1/n^(1/2). The shot noise is thus reduced at a slower rate than thereduction in the photocurrent, and the S/N is accordingly reduced.

The present invention is effective in application at room temperature.The input circuit for reading out of the signal detected by thephotodetector is not limited to a J FET, and the switch S1 is notlimited to a MOS FET or pn-junction element. For this reason, the scopeof the present invention covers the following structure.

This photodetection device consists of a photodetector for the detectionof incident light, a signal-readout element for reading out the signaldetected by the photodetector, an amplifier for amplification of thereadout signal of the signal-readout element, a feed-back circuit forfeeding the output of the amplifier back to the gate of thesignal-readout element through a feed-back capacitor; a reset circuitused to reset the feed-back capacitor by discharging the feed-backcapacitor through a first reset switch; a second reset switch connectedbetween the first reset switch and the feed-back capacitor; the firstswitch is connected between the gate of the signal-readout element andthe output of the amplifier through the second reset switch. Inaddition, control is comprised of control contributed by the firstswitch and the second switch. Control means control of the first andsecond reset switches such that the first reset switch is turned “on”and then “off” while the second reset switch is “on”. The second switchis only turned “off” after the first reset switch has been turned “off”.The feed-back capacitor is reset while the first and second resetswitches are “on”.

The other present invention is a photo-detection method for aphotodetection device, and is composed as follows. This photodetectiondevice consists of a photodetector for the detection of incident light,a signal-readout element for reading out the signal detected by thephotodetector, an amplifier for amplification of the readout signal ofthe signal-readout element, a feed-back circuit for feeding the outputof the amplifier back to the gate of the signal-readout element througha feed-back capacitor; a reset circuit used to reset the feed-backcapacitor by discharging the feed-back capacitor through a first resetswitch; a second reset switch connected between the first reset switchand the feed-back capacitor; the first switch is connected between thegate of the signal-readout element and the output of the amplifierthrough the second reset switch. The first reset switch and the secondreset switch are controlled so that the first reset switch is turned“on” and “off” while the second reset switch is “on”. The second switchis turned “off” after the first reset switch has been turned “off”. Thefeed-back capacitor is reset while the first and second reset switchesare “on”.

The objectives, advantages and features of the present invention will bemore clearly understood with reference to the following detaileddisclosure and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing that gives an example of the circuit ofphotodetection device A of the present invention.

FIG. 2 shows drawings of timing charts of the reset in the circuitexample of the photodetection device A of the present invention.

FIG. 3 is a drawing that gives an example of the circuit ofphotodetection device B of the present invention.

FIG. 4 is a drawing that gives an example of the circuit ofphotodetection device C of the present invention.

FIG. 5 is a drawing that gives an example of the circuit ofphotodetection device D of the present invention.

FIG. 6 is a drawing that gives an example of the circuit ofphotodetection device E of the present invention.

FIG. 7 is a drawing of an example of a circuit of the prior art in photosensing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(i) Explanation of the Example Circuit for Photodetection Device A.

FIG. 1 shows an example of the circuit of the photodetection device A,in which noise is reduced to the utmost limit as much as possible by theuse of the CTIA circuit, which was explained as the prior art. Theexplanation of photodetection device A is as follows.

Photodetection device A is comprised of the photodetector 1 (for examplea photo-diode) for detecting incident light, the input J FET 2 forreading out of the sensing signal detected by the photodetector, the opamp. 3 for amplifying the signal detected by the input J FET 2, thefeed-back circuit for feeding of the output of the op amp. 3 back to thegate of the input J FET through the feed-back capacitor 4, the resetcircuit for resetting feed-back capacitor 4 by discharging it throughthe reset MOS FET S1 and a circuit formed by a switch and resistor R1.Voltage at the same level as the gate voltage V_(G) of the input J FET 2is applied to the source (or drain) through resistor 2, and the circuitof switch S2 and resistor 2 is connected between the source (or drain)of the reset MOS FET S1 and the feed-back capacitor 4. The reset MOS FETS1 and switch S2 are controlled so that the reset MOS FET S1 is turned“on” and “off” while the switch S2 is “on”. The switch S2 is turned“off” after the reset MOS FET has been turned “off”. Resetting of thefeed-back capacitor is performed while the reset MOS FET and switch S2are “on”.

In this circuit, reset pulses are applied by an external circuit to thegate of the reset MOS FET S1. In the example, the pulse-generatingcircuit and control circuit (not shown in the figure) are configured soas to generate and supply pulses with an appropriate timing to the gateof the reset MOS FET S1. The pulses generated by the pulse-generatingcircuit are passed through an LPF (low-pass filter) to reduce theirhigh-frequency components, and are then supplied to the gate of thereset MOS FET S1.

Furthermore, the switch S2 can take the form of a mechanical switch orelectro-magnetic relay etc., or can be realized as a transistor switch.A photodiode or some known photodetector other than a photo-diode can beused as the photodetector 1.

In the above-mentioned example of the circuit of photodetection deviceA, the reset circuit is only inserted in the feed-back circuit while thereset is being applied; in charging of the feed-back capacitor, thesource voltage of the reset MOS FET S1 is kept at the same level as thegate voltage of the input J FET 2. In the prior art, on the other hand,the reset MOS FET S1 is always connected in parallel with the feed-backcapacitor. The voltage between the electrodes of the feed-back capacitor4 is thus always directly applied across the source and drain of thereset MOS FET S1, and thus induces leakage current.

This leakage current is inevitable with the prior art, since somevoltage is applied across the source and drain even when the reset MOSFET S1 is “off”. In the present invention, however, this leakage currentis avoided by having the source (or drain) voltage of the reset MOS FETS1 at the same voltage level as the gate voltage of the input J FET 2.

The switch S2 separates the reset MOS FET S1 from the output of theamplifier during charging of the feed-back capacitor. The resistors R1and R2 divide the output of the op amp. 3 and reduce the voltage appliedto the source. Reducing the voltage applied to the source avoids theeffect of a reset inducing large fluctuations on the source of the resetMOS FET S1.

To make the reset MOS FET S1 turn “on” for a reset, a level of voltagewhich varies with the source voltage has to be applied to the gate(whether the voltage is positive or negative and the voltage valuediffers from FET to FET). The source of the reset MOS FET S1 isconnected to the output of the amplifier, so that the source voltagevaries with the output voltage of the amplifier. The resistors R1 and R2are inserted to prevent the induction of large fluctuations at thesource of the reset MOS FET S1 by variations in the voltage output bythe op amp. 3.

With regard to gate-source voltage, the source voltage during resetoperation is low enough that the reset MOS FET S1 is certain to be “on”at any output voltage of the op amp. 3. For example, if the gate-sourcevoltage at which the reset MOS FET S1 is “on” is about −3 V, the sourcevoltage in the reset action has to be at least −0.1 V.

As the maximum output voltage of the op amp. is set to ±18 V, theresistors R1 and R2 are selected so that 1/100 of the voltage output bythe op amp. 3 is applied to the source during the reset action. Inaddition, the resistor R1 has to be sufficiently larger than the “on”resistance of S2. It would be possible for another system to apply thenecessary voltage to the gate of the reset MOS FET S1, taking account ofvariation in the source voltage of the reset MOS FET S1. Theabove-mentioned system is the simplest.

The maximum output voltage of an op amp. is generally determined by thevoltage of the power supply for the op amp. If amplification is to beapplied by a stage after op amp. 3, the output voltage of the op amp. 3has to be lower than the maximum output voltage so that the output ofthe final amplifier does not go into saturation. That is, the maximumoutput of the CTIA circuit should be 1/(maximum output amplification atthe final stage). The ratio of R1 to R2 thus does not have to beextremely large.

However, the output of the CTIA circuit may occasionally becomes themaximum voltage because of strong incident light. If we consider suchcases, the R1 and R2 values have to be decided on the basis of thecharacteristic maximum voltage for the op amp. used in the CTIA circuit.

Application of the resistors has the following merits. As the switch S2is inserted in series with the resistor R1, its “on” and “off”resistance affects the division ratio of the output of the op amp.However, as long as the “off” resistance of the switch S2 issufficiently greater than the resistor R1, the voltage applied to thesource of the reset MOS FET S1 remains sufficiently low that the leakagecurrent during discharge is negligible.

Thus, switch S2 need not have an extremely high “off” resistance. It canthus be kept at room temperature (the “off” resistance is far lower atroom temperature than at low temperatures). This is important when thereset circuit is one in which a reset MOS FET is not used, as is laterexplained.

The photodetection device of the present invention is also performedwith a circuit such that the source and drain are exchanged in FIG. 1.That is, the circuit that the reset MOS FET is connected to the gate ofthe input J FET 2 and the drain of this MOS FET is connected to theswitch S2 and resistor R2.

FIG. 2 gives timing charts for the example circuit of photodetectiondevice A. In FIG. 2, S1-on pulses are pulses to turn the reset MOS FETS1 for resetting “on”, and S2-on pulses are pulses to turn the switch S2“on”. Turning the reset MOS FET S1 and switch S2 “on” and “off” must becontrolled timely for the CTIA circuit to operate stably. The timingchart is given in FIG. 2.

Careful attention is necessary in using a J FET as the element forreading out of the photodetector in the CTIA circuit of FIG. 1. As wellas the photodetector itself and the reset MOS FET S1, the input J FET 2contributes leakage current. However, this leakage current diminishesover time, with the noise level dropping below 10⁻¹⁸ after from a few toten hours.

While this is so, applying a voltage other than the voltage which hadbeen across the gate and drain or gate and source of the input J FET 2increases the leakage current. The time this leakage current takes todisappear, i.e., for the suddenly increased leakage current to return tothe original leakage current, is the same as was described above. Suchvoltage changes happen at the time of resetting in particular. Inaddition, when the edge of the reset pulse becomes sharp enough, the opamp. is unable to follow the pulse because the voltage change is toosudden and the input J FET 2 induces a DC-voltage drift.

These problems are solved by always keeping the voltage applied to theinput J FET 2 at the same level, even keeping it at the voltage beforethe reset through a reset. The switch S2 has to be “off” for charging ofthe feed-back capacitor and “on” for a reset. However if the reset MOSFET S1 is “on” before the switch S2 is switched “on” in a reset, thevoltage Vg is applied to the input gate of the input J FET 2 through thereset MOS FET S1 and resistor R2, because the reset MOS FET S1 is notconnected to the feed-back circuit.

As the result, the op amp. 3 goes into saturation in either the positiveor negative direction, because the feed-back from the op amp. 3 to thegate is not being carried out. After this, when the switch S2 is turned“on”, the feed-back circuit is connected to the reset MOS FET S1. Inthat situation, some time is needed for the op amp. 3 to recover fromits saturation state; during this time, the gate voltage varies greatlyover a short period. This fluctuation causes problems.

The reset MOS FET S1 and the switch S2 are thus controlled in the wayshown in the timing chart. As is shown in FIG. 2, it is important thatthe switch S2 is already “on” when the reset MOS FET S1 is turned “on”.

When control is in this way, the op amp. 3 always controls the circuit,regardless of the state of the reset MOS FET S1 for resetting, so thegate voltage of the input J FET 2 for readout can be kept constant.

A significant feature of the time chart shown in FIG. 2 is that therises and falls are not sharp. Let us consider the pulses of the timingchart in FIG. 2; if the pulses which control the switch 2 rise toosharply, the pulses induce a variation in the voltages of J FET 2. Whenthe pulses are applied to the control part of the switch S2 (if it is aMOS FET, its gate), the signal is applied to the CTIA circuit throughthe feed-back capacitor. If the signal change is too rapid, the op amp.3 will be unable to follow the signal. This causes some problems.

(ii) Explanation of the Example Circuit of Photodetection Device B

FIG. 3 shows an example of the circuit of photodetection device B. As isshown in FIG. 3, the circuit example of the photodetection device B iscomprised of photodetector 1 for the detection of incident light, inputJ FET 2 for reading out of the sensing signal from the photodetector, opamp. 3 for amplifying the signal detected by the input J FET 2, afeed-back circuit for feeding of the output of the amplifier back to thegate of the input J FET 2 through a feed-back capacitor 4, a resetcircuit for resetting by discharging the charge of the feed-backcapacitor 4 through a reset pn-junction element S3 and a circuit made upof a switch and resistor R2. The same level of voltage as the gatevoltage V_(G) of the input J FET 2 is applied to the reset pn-junctionelement S3 through the resistor R2, and the circuit of switch S2 andresistor R1 are connected between the reset pn-junction element 3 forresetting of the feed-back capacitor. The circuit has feature that,following turning the second switch to “on”, the first switch turnsautomatically to “on”.

The photodetection device B improves on some problems which arise withthe photodetection device A and are described below. In the presentinvention, the polarization noise comes to determine the limit ofdetection for the incident light as the capacitance of the photodetectorfalls. The polarization noise occurs because of fluctuations in thedielectric polarization of matter, and increases in proportion todielectric loss.

As the dielectric loss is a kind of resistive loss, it acts in the sameway as the thermal noise (Johnson noise) that occurs in a resistor. Thepolarization noise is generated in the gate-circuit elements of theinput J FET 2, that is, all elements and materials that are connected tothe gate of the input J FET 2, such as the sensor 1 and the input J FET2 itself.

Of the elements of the gate-circuit of the photodetection device A, thereset MOS FET S1 is the greatest contributor of noise. This is becauseimperfect crystallization of the film of silicon oxide or nitride whichseparates the gate of the MOS FET from its channel leads to increaseddielectric loss. The reset MOS FET S1 used in the photodetection deviceA generates the polarization noise of more than two times of allpolarization noise generated by elements other than the MOS FET S1.

Thus, in circuit example 2, we eliminate the reset MOS FET S1 which wasused as the switch S1 in the photodetection device A, and replace thisreset MOS FET S1 with a reset pn-junction element. Either a J FET orphotodiode may provide this reset pn-junction element. A J FET can beused as the pn-junction element when a pair of its electrodes have beenconnected with each other. It is, however, best to use as small acapacitance as is possible. This is because a lower capacitance isaccompanied by less polarization noise and leakage current.

With the exchange of the reset MOS FET S1 for a reset pn-junctionelement, the reset circuit changes to become as shown in FIG. 3. Ascurrent does not flow in the pn-junction element unless a forwardvoltage is applied across it, the p side of the pn-junction element S3is connected to the output of op amp. 3 when the output of the op amp. 3enters the positive voltage range during charging of the feed-backcapacitor 4, as is shown in FIG. 3. Whether a pn-junction orPIN-junction is used as the pn-junction element, its direction is thesame as the direction of photodiode 1 with respect to the gate of the JFET (see FIG. 3).

In addition, current does not flow in the reset pn-junction element S3unless the applied voltage is greater than some fixed positive voltage.For example, applying a voltage of somewhat more than 0.5 V in theforward direction is necessary for a silicon element; R1 and R2, whichdivide the output of op amp. 3, should thus be selected so that thevoltage applied to the element's p-side is not too low.

The value is determined on the basis of the post-reset voltage desiredfor the output of op amp. Now let us consider to make the gate voltageVg to 0V, and the output of the op amp. 3 1V. When the gate voltage is0V, the output voltage of the op amp. 3 is about 0.5*(R1+R2)/R2. Thus,the resistors are selected as R1=R2. In this way, the ratio of thevalues R1 and R2 is not important, however the absolute value isimportant. Because too small the value leads to over-current flowing inthe pn-junction element S3 for resetting, and +may damage the elements.

This method of resetting has other merits. The reset pulses required forthe photodetection device A are not required with the reset pn-junctionelement S3 for resetting, so the circuit is simpler. Because, in thecircuit, following turning the second switch into “on”, the first switchturns automatically to “on”. The reset pulse is not needed thus, and thecircuit is simplified.

The photodetection devices of the present invention are used to detectrays of wide wavelength range such as from far infrared rays to visiblerays. The light detecting sensitivity is high in measurement atcryogenic temperature. Especially the photodetector is cooled down tothe liquid Helium temperature for making its sensitivity high, whendetecting the far infrared rays. A photodetector made of compoundsemiconductor is suitable for measuring in liquid Helium temperature.

The photodetector 1 and the input J-FET are used usually in a bodyunited each other. When the material of the photodetector 1 is differentwith that of the J-FET 2, the substrate of the photodetector 1 is unitedback to back with that of the J-FET by using the technique of directhybridization.

When each of the input J-FET, the pn-junction element S3 and the switchS3 is made of the same material, all the elements may be mounted on asubstrate. This is very convenient to make a two-dimensional arrayphotodetector. Because the substrate of photodetector and the sharedsubstrate of the three elements are united into one body easily with thedirect hybridization.

This is because the sensitivity of a long-wavelength photodetector isnot high unless it is kept at cryogenic temperatures. While a GaAs J FETcan operate at cryogenic temperatures, a MOS FET cannot be made of GaAs.Using a GaAs J FET as the reset switch S1 may thus be considered;however, the “off” resistance of a GaAs J FET is far lower than that ofMOS-type FETs in general. For this reason, the GaAs J FET cannot be usedas the reset switch of a prior art CTIA circuit.

The switch S2 is inserted in the circuit only to make the pn-junctionelement S3 “off” in time for charging of the feed-back capacitor. Forthis reason, high off resistance of the switch S2 is not needed and a JFET is usable as the switch S2. So the switch S2 may be composed of thesame material with the input J-FET 2 and the pn-junction element S3.Thus all of the elements are made of the compound semiconductor such asGaAs to integrate the elements, which makes fabrication of thephotodetection device very easy, as mentioned above.

(iii) Explanation of the Example Circuit of Photodetection Device C

FIG. 4 shows an example of the circuit of the photodetection device C.In the example of the circuit for photodetection device C, the GaAs JFET is used as the input J FET 2 and switch S2, with a GaAs pn-junctionelement S4 as the reset pn-junction element S3. Further points regardingthis embodiment are explained in the following passages.

As is shown in FIG. 4, the example circuit of the photodetection deviceC is comprised of a photodetector 1 for detecting incident light, aninput GaAs J FET 5 for reading out of the sensing signal from thephotodetector, an op amp. 3 for amplifying the signal detected by theinput GaAs J FET, a feed-back circuit for feeding of the output of theop amp. 3 back to the gate of the input GaAs J FET 5 through a feed-backcapacitor 4, a reset circuit for resetting feed-back capacitor 4 bydischarging its charge through a reset GaAs pn-junction element S4 forresetting, and a circuit of switch S2 and resistor R1. Voltage at thesame level as the gate voltage V_(G) of the input GaAs J FET 5 isapplied to S4 through a resistor R2, and the switch S2 composed of aGaAs J FET is connected between the S4 and, via the resistor R1, to thefeed-back capacitor 4.

The input GaAs J FET can operate at temperatures below liquid heliumtemperature (4.2 K). Sensitivity is thus improved by using an input GaAsJ FET 5 for reading out of the signals detected by the photodetector,which is highly sensitive to long-wavelength light, and also by usingthe GaAs J FET as the switch S2 and by having operation at cryogenictemperatures.

In addition, when GaAs pn-junction element S4 is used as the resetswitch of the reset circuit, all of the circuit elements can be made ofGaAs. In this case, fabrication of the photodetection device C is easedby using the same techniques for all of the GaAs elements. Since thereset can be carried out by simply applying forward voltage to the GaAspn-junction element S4, and following turning the switch S2 to “on”, theforward voltage is automatically applied. Thus the reset circuit isfurther simplified.

In the explanation given above, when the op amp. 3 does not work at acryogenic temperature, all the elements other than the op amp. 3 areplaced at a cryogenic temperature. When the op amp. 3 works at acryogenic temperature, all the elements including the op amp. 3 areplaced at a cryogenic temperature. However, the photodetector, inputGaAs J FET 5, GaAs J FET switch S2 and GaAs switch S1 can operate alsoat a room temperatures in the present invention. The inventor of thepresent invention found that, even when all of the elements are placedat room temperature, the circuit operates and produces fine results,just as it does in cryogenic operation.

A reset circuit in which the pn-junction element is used as the resetswitch is not only applicable to CTIA circuits, but is also applicableto CIA circuits. In a CIA circuit, reading of voltage generated in thephotodetector is by a FET for reading out of the detected signals; thefeed-back circuit is not used in the CTIA circuit (see FIG. 5). In thiscase, the reset can be carried out by applying a forward voltage to thereset pn-junction element.

(iv) Explanation of the Example Circuit of Photodetection Device D

FIG. 5 shows an example of the circuit of the photodetection device D.The photodetection device D is comprised of a photodetector 1 fordetecting incident light, a J FET 2 for reading out of the detectionsignals from photodetector 1, a pn-junction element S3, which isconnected to the gate of the input J FET 2 and is for resetting ofcharge accumulation in the input J FET 2, and a switch 6. The switch 6controls to turn the pn-junction element S3 to “on” or “off”. The meansof control for control of the switch S6 is not shown in FIG. 5.

In this case the photodetector 1 and reset pn-junction element S3 areconnected mutually at the same-polarity terminal, and the photodetector1 and reset pn-junction element S3 are connected in parallel between thegate of the input J FET 2 and GND.

In reading of the detected signals, the switch 6 is connected to its GNDelectric potential (earth potential) side to turn the pn-junctionelement S3 “off”. In resetting, the switch 6 is connected to the voltageV side to turn the pn-junction element S3 “on”. When the pn-junctionelement S3 for resetting is turned “on” in this way, the chargeaccumulated in the capacity of the pn junction of photodetector 1 and/orthe gate circuit of the J FET 2 is discharged and the circuit is reset.

(v) Example Circuit of the Photodetection Device E.

FIG. 6 shows an example of the circuit of the photodetection device E.The example circuit of the photodetection device E reduces noise in thecase where a large-capacitance photodetector is used in any of theabove-mentioned photodetection devices, A-D.

As was mentioned above, the polarization noise is dominant when thecapacitance of the photodetector 1 is small. However, the noise of theinput J FET 2 (see circuit examples 1, 2, and 4) and of the input GaAs JFET 5 (see circuit example 3) becomes a problem when the capacitance ofthe photodetector 1 is large.

If the capacitance of the photodetector 1 is C, and the noise voltage ofthe circuit for reading out of the detected signals is Vn, the noisecurrent of the readout circuit is ωC·Vn. Noise thus increases inproportion to the capacitance of the photodetector 1. Thus, when thecapacitance of the photodetector 1 increases, the noise (input-referrednoise, henceforth referred to as noise) of the J FET is dominant.

Under this condition, enlarging the size of photodetecting area of thephotodetector to increase the amount of light incident on thephotodetector 1 does not improve the S/N ratio at all, because of theproportionate increase in capacitance. The present invention provides aserial connection device of the photodetectors which resolves thisproblem.

Here, we prepare a photodetector A with a photo-sensing area S/n that ofa standard photodetector B, the detection-surface area of which is S,placing n such photodetectors A parallel with each other and thenelectrically connecting them in series with each other. In this case,the capacitance of one photodetector is 1/n of the original one; as theyare connected in series with each other, the capacitance is reduced toC/n², where C is the capacitance of the photodetector B.

The amount of incident light on each photodetector is 1/n of the amountfor the original one; when they are connected in series with each other,the photo current of each of the n photodetectors thus connected is 1/n,and the total light-incident area is the same as for the photodetectorB. When we consider the noise as separate J FET noise and polarizationnoise components, we see that the J FET noise is reduced to 1/n² of theoriginal value, i.e., in proportion to the reduction in capacitance.

However, the polarization noise is reduced at the rate of 1/n, becausethe polarization noise is proportional to the root of the capacitance.Increasing n thus reduces the noise of the J FET more rapidly than thephoto-current, which improves the S/N. With increasing “n”, however, thenoise of the J FET becomes smaller than the polarization noise at some“n”, and the polarization noise then becomes the dominant form of noise.In that situation, the reduction in noise is at the rate of 1/n. Sincethe photo-current is also reduced at the rate of 1/n, the S/N ratio doesnot change. That is, this method is effective in those cases where thenoise of the J FET is dominant. Furthermore, under this condition, theleakage current and shot noise are reduced at the rate of 1/n and1/n^(1/2) respectively, so there is less reduction in the noise than inthe photo-current, and the S/N is reduced. The situation for thephoto-current is the same, so the method has the opposite effect whenthe leakage current or shot noise of the photo-current is dominant.

FIG. 6 shows an embodiment of what is described above. In FIG. 6, (a)shows a prior-art photodetector (see FIG. 7), (b) shows the area of thephotodetector 1 divided up into n devices (that is, preparing deviceseach having a light-incident area of 1/n), (c) shows the placement ofthe photodetectors parallel with each other, and (d) shows the serieselectrical connection of the n photodetectors. The characteristics of(a)-(d) are as follows.

(1) Concerning area, let the area in (a) be S (mm²) and the area in (b)be S/n (mm²/unit) per photodetector; the total area in (c) and (d) isthen S(mm²).

(2) Concerning capacitance, let the value in (a) be C (pF) and thecapacitance in (b) be C/n (pF/unit) per photodetector; the totalcapacitance in (c) and (d) is then C/n² (pF).

(3) Concerning photo-current, let the current in (a) be I(A) and thecurrent in (2) be I/n (A/unit) per photodetector; the total current in(c) and (d) is then I/n (A).

(4) Concerning FET noise, let the noise in (a) be I_(FET) (A) and thenoise in (b) be I_(FET/n) (A/unit) per photodetector; the total noise in(c) and (d) is then I_(FET)/n² (A).

(5) Concerning polarization noise, let the noise in (c) be I_(P) (A) andthe noise in (b) be I_(P)/n^(1/2) (A) per photodetector; the total noisein (c) and (d) is then I_(P)/n (A).

(6) Concerning shot noise, let the noise in (a) be I_(P) (A) and thenoise in (b) be I_(S)/n^(1/2) (A) per photodetector; the total noise in(c) and (d) is then I_(P)/n^(1/2) (A).

The n photodetectors are placed so that the input light is guided to thelight-incident area of each photodetector and the photodetectors areelectrically connected in series with each other to realize thephotodection device shown in FIG. 6. Otherwise, n photo-incident areasmay be formed on one substrate and electrically connected in series toact as one sensor.

In the example circuits shown in FIG. 1-FIG. 6, elements other than opamp. 3, that is, photodetector 1, J FET 2, the switch S1 or S5 and theswitch S2, and resistors R1 and R2 etc., are used in a cryogenic vesselthat is cooled to a cryogenic temperature (for example, 77K). Forexample, op amp. 3 is kept at room temperature but the other devices areplaced in a liquid-nitrogen cryostat.

However, a situation where some of these other devices, such as theswitch S2 and op amp. 3, are at room temperature while the otherelements are at cryogenic temperature is also allowed. Furthermore, evenwhen all of the elements are at room temperature, effective noisereduction is realized.

The many features and advantages of the present invention are apparentfrom the detailed specification and the appended claims are thusintended to cover all such features and advantages of the invention asfall within the true spirit and scope of the invention. Further, sincenumerous modifications and changes will readily occur to those skilledin the art, it is not desired to limit the invention to the exactconstructions and operations that have been illustrated and described;accordingly, all suitable modifications and equivalents which fallwithin the scope of the invention may be included in the presentinvention.

1. The photodetection device comprising: n photodetectors, wherein nincident planes of the photodetectors are placed parallel with eachother and electrically connected in series with each other.